Hydrogels for Stem Cell Fate Control and Delivery in Regenerative Medicine - In-Situ Gelling Polymers: For Biomedical Applications - page 187

Fig. 2 Synthesis scheme of glycol chitin. Reproduced with permission [ 52 ]. Copyright 2013,
Elsevier
3.2 Synthetic Hydrogels
3.2.1 Synthetic Gels Prepared by Ring-Opening Polymerization
Biodegradable polymers such as poly( ʵ -caprolactone) (PCL) and poly(lactic acid)
(PLA) are typically synthetically prepared by ring-opening polymerization of
lactones. The basic monomeric unit of PLA is lactic acid. In theory, the esterifi-
cation of lactic acid forms PLA and the reaction can be expected to be straight-
forward. However, this is an equilibrium reaction and it is difficult to remove the
water produced by the esterification process completely. The bonds formed are
easily hydrolyzed by the water molecules and the maximum attainable molecular
weight is thus limited. This problem is solved by using a cyclic lactide. This lac-
tide undergoes a ring-opening polymerization to give PLA. As there is no water
molecules produced in the course of the reaction, the molecular weights that can
be attained are much higher than typical polycondensation reactions. Ring open-
ing polymerization has been used by Jeong et al. to prepare thermogelling pol-
ymers. Using a telechelic dihydroxyl compound as the initiator, either a small
molecule glycol or poly(ethylene glycol), many polyesters can be synthesized in
this manner. In particular, polymers such as PCL, [ 55 - 59 ] PLGA, [ 59 - 67 ] and
PLA [ 68 , 69 ] have been reported. Poly(amino acids) have also been polymerized
by ring-opening polymerization of N-carboxyanhydrides (NCA). To date, poly
(l-Ala-co-L-Phe)-poly(propylene glycol)-poly (ethylene glycol)-poly(propylene
glycol)-poly(L-Ala-co-L-Phe), poly(ethylene glycol)-poly(L-alanine-co-L-
phenyl alanine), poly-(ethylene glycol)-poly(D-alanine-co-D-phenyl alanine),
poly(ethylene glycol)-poly(L-alanine-co-L-phenyl alanine) grafted chitosan have
been demonstrated to have a thermogelation effect [ 70 - 73 ].
3.2.2 Synthetic Gels Prepared by Radical Polymerization
Radical polymerization is the workhorse of many polymer chemists and has been
used extensively to fabricate materials, which are easily scalable and also having
multiple characteristics due to the different monomers in the polymer backbone.
Typical monomers polymerized include N-isopropylacrylamide (NIPAAm) and